Flow control valves are used to control the speed of hydraulic actuators. However, these flow control valves also contribute to considerable heat development. In case of fixed displacement pump circuits, the actuators require varying flow rates during operation, hence these pumps must be sized to deliver the highest flow required. But unfortunately, when less flow is required, the excess flow from these pump must be "dumped" over the system relief valve at maximum system pressure. This process converts the unwanted energy directly into heat. For this reason fixed displacement pumps should only be used in constant speed circuit applications, or in circuits where speed control is of very short durations, such as end-cushioning or short-load deceleration.
Nevertheless, there are still applications where fixed displacement pumps are used. But it is essential that the fixed displacement pump is sized precisely for the speed or the flow rate required.
As a thumb rule, a fixed displacement pump or open center system is adequate only if none of the following statements hold true:
- System pressure must be maintained on a stalled actuator.
- The hydraulic circuit operates over a broad speed range.
- The pump cannot be unloaded by the circuit design during idle periods.
- During a large portion of the cycle the actuator must be operated at a very low speed.
Another important issue is the efficient use of available energy resources. Thus, engineers should carefully evaluate the power requirements of new machines and appliances, and should constantly look for new ways to reduce energy demands to the lowest possible level.
Using energy-saving, variable displacement pumps has contributed greatly to overcoming the existing stigma that hydraulic systems are by structure inefficient. These variable displacement pumps only deliver flow when and as required by the system.
The variable displacement pumps provides an additional benefit as heat is not generated by moving oil around the circuit when no actuator work is being done. Even when a fixed displacement pump is being unloaded (with a tandem center valve), energy is converted to heat, the simple reason being that the oil is in motion. On the other hand, the variable displacement pump can be controlled to produce energy only when it is required. Thus, when no heat is developed, no energy is wasted. Moreover, variable displacement pumps often eliminate or reduce the need for flow control and pressure reducing valves, and thus these pumps cost much more.